 Ionic substances have a number of characteristic properties that's due to their bonding. They have high melting points, they tend to be brittle, and they conduct electricity when they're a liquid or when they're dissolved in water, but not when they're solids. So let's look at melting points first, melting and boiling points in fact, but we'll just deal specifically with melting points. Ionic substances tend to have high melting points because Ionic bonds are very strong, which means they require a lot of energy to break. What's more, salts that are made of ions with higher charges tend to have higher melting points. So as you can see in this graph here, the melting point of sodium chloride, which is made up of sodium and chloride with single charges, is much lower than the melting point of magnesium oxide where the ions have 2 plus and 2 minus charges. Ionic substances are brittle, which means that when you hit them they shatter rather than squashing. This is because if the crystal lattice shifts or is disrupted, positive ions will line up with positive ions which repel each other, and that causes the lattice to break. We usually think of electricity as being electrons moving through a wire. In fact, electricity can be conducted by anything that has a charge and is able to move. When a salt is in its solid form, the ions are fixed in place in the lattice. This means that they cannot conduct electricity because although the charge carriers, the ions, are vibrating, they're unable to move from one place to another. However, when the salt is melted it becomes conductive. This animation from Vizchem shows the melting of a sodium chloride crystal. When the ions get hot enough, they're freed from their lattice and they can move around. If you put two electrodes in a molten salt, the positive ions will travel in one direction and the negative ions will travel in the other and so the current is conducted. Salt can also conduct electricity when they're dissolved in water. This is another of Vizchem's animations and shows an aqueous sodium chloride solution. Notice that the gray sodium ions and the green chloride ions can move freely among the water molecules. And they can move, like in the molten salt, they are able to conduct electricity.